1. Field of the Invention
The present invention relates to a process for the separation and recovery of an unsaturated hydrocarbon using a facilitated transport liquid membrane system, and more particularly concerns the aforesaid process with purification of the unsaturated hydrocarbon.
2. Discussion of the Prior Art
There currently exist a number of methods for the selective separation of gaseous feed stream components, including the removal of an unsaturated hydrocarbon from a gas stream containing a mixture of the unsaturated hydrocarbon with a saturated hydrocarbon.
Facilitated transport membrane technology is a known separation technique. It has been demonstrated in the laboratory for the selective separation of a gas stream component, such as the removal of an unsaturated hydrocarbon from a feed stream containing a mixture of the unsaturated hydrocarbon and at least one other material. The agent used normally to facilitate the transport is a complexation agent that contains a metal ion that has an affinity for the unsaturated hydrocarbon to be separated. The silver ion is known to be especially useful for the separation of olefins.
Sirkar, U.S. Pat. No. 4,750,918 discloses a type of facilitated transport involving the use of hollow fiber membranes, as opposed to flat sheet membranes. In this patent, the feed and recovery hollow fibers are immersed in a liquid bath to avoid drying problems often encountered with flat sheet immobilized membranes. The gases permeate through the wall of the feed hollow fiber, diffuse across the liquid bath and permeate into the bore of the recovery hollow fiber.
S. Majumdar et al., "A New Liquid Membrane Technique for Gas Separation," A. I. Ch. E. Journal, Vol. 34., No. 7, pages 1135-1145, discloses a liquid membrane separation technique for gas mixtures in which feed and sweep gases flow through the lumen of two different sets of hydrophobic microporous hollow fibers while a liquid on the shell side acts as the membrane. The technique involves the use of a dense population of hydrophobic microporous hollow fibers of small outside diameter and a permeator shell. In such a hollow-fiber assembly, the space between the adjacent fibers is filled with an aqueous liquid chosen to function as a liquid membrane. One set of hollow fibers carries the high-pressure feed gas while a second set of hollow fibers carries a sweep gas, usually at a pressure considerably lower than the feed pressure. Ideally, the fiber bundle is arranged in such a way that a feed gas-carrying fiber is immediately adjacent to a sweep gas-carrying fiber. Water and aqueous electrolytic solutions do not penetrate the pores of these hydrophobic fibers unless the liquid pressure exceeds 10-15 atmospheres. The feed gas, species contact the membrane liquid at the pore mouths at the outside surface of the feed fiber. They dissolve at this feed gas membrane liquid interface and diffuse through the liquid membrane to the open pores at the outside surface of the nearest sweep fiber, where they desorb. The desorbed gases are carried away through the sweep fiber lumina by an inert sweep gas. The membrane liquid between the fibers is usually stationary. Z. Qi et. al., "Microporous Hollow Fibers for Gas Absorption," J. Membrane Sci., Vol. 23, page 321 (1985), also discloses the use of two hydrophobic microporous hollow-fiber modules for gas separation, one in the sorption mode and the other in the desorption mode.
R. Creusen et al., European Patent Application No. 94201995.1, filed on Jul. 11, 1994 and published on Jan. 18, 1995 as Publication No. 0634204 A1, discloses a process and device for the separation of an unsaturated hydrocarbon from a fluid mixture containing saturated hydrocarbons in which in a first stage the fluid mixture is passed at superatmospheric pressure to one side of a first semiselective gas separation membrane with a non-porous active layer, and a liquid complexing agent is passed along the other side of such first membrane, where the unsaturated hydrocarbon is bound through complexation at the interface of the membrane and the complexing agent. In a second stage, the unsaturated hydrocarbon is dissociated from the complexing agent through a temperature increase, and the resulting mixture of the complexing agent and dissociated unsaturated hydrocarbon is separated, and the complexing agent is then recycled. Preferably the mixture of complexing agent and dissociated, unsaturated hydrocarbon is passed at superatmospheric pressure to one side of a second semiselective membrane with a non-porous active layer wherein the unsaturated hydrocarbon migrates to the other side of the second membrane and is discharged. The disclosed advantage of such process and device is that the process of sorption involving the complexation reaction in the first stage is separate and independently adjustable from the process of desorption involving the dissociation reaction in the second stage and that a substantial dissociation of the complex formed can be obtained by using a temperature increase.
However, while there has been interest in using membranes as a means of separation, the lack of highly selective membranes has hindered the application of membranes for this purpose. The problem associated with all facilitated transport processes is that materials which do not form complexes with the complexation agent but which are at least somewhat soluble in the liquid solution of the complexing agent, as well as the unsaturated hydrocarbons that actually form complexes with such agent, are removed from the fluid feed mixture in the sorption process and recovered with the unsaturated hydrocarbons in the dissociation process. Therefore, facilitated transport processes in general are less selective for the separation and recovery of the unsaturated hydrocarbons than the chemistry alone would suggest. In addition, more than one unsaturated hydrocarbon may form complexes with the complexing agent. Consequently, it is highly desirable to improve the selectivity for the recovery of each separated unsaturated hydrocarbon in facilitated transport processes without an unacceptably large reduction in the yield of unsaturated hydrocarbons recovered.